Fumed silica can be manufactured by a process involving reacting evaporated silicon tetrachloride in an oxygen-hydrogen flame to produce silicon dioxide and hydrochloric acid. The resulting product is 99.8% pure silica, its particles are non-porous, extremely small in diameter (average 12 millimicrons), have a defined shape, and are loosely agglomerated. The surface of the particles contain siloxane and silanol groups. In terms of numbers, the siloxane groups within the resulting product predominate causing the product to have a fully inert character. The silanol groups within the resulting product impart hydrophilic properties. However, with compounds that are especially reactive, such as organosilanes, chemical reactions with the silanol groups may be carried out, thus imparting hydrophobic properties. Areas of application for the resulting product include paints and inks, and use as a suspension and/or and anti-blocking agent.
The manufacture of precipitated silica begins with the reaction of an alkaline silicate solution, usually sodium silicate (waterglass), with a mineral acid. The resultant white precipitate is pumped through filter presses which yield cakes with higher solids content than could be obtained through other filtration means. Salts that are formed during the precipitation process are washed out in the filtering equipment. The resulting solid content of the filtered cakes is generally in the range of 15 to 20%. Drying is carried out through turbine, rotary, or spray drying. Since 80 to 85% of the water must be driven off during the drying process, this process is cost intensive. Milling can generally be done after drying to reduce the size of agglomerates formed during the drying process. After milling has been completed, sizing classification separates the product from grit and other impurities. Purity of the resulting product is in the range of 98-100% silica. Areas of application for precipitated silica include use as an agent for any of the following functions: anti-blocking, anti-slip, defoaming, insulation, thickening, polishing, carrier (liquids to powders), filler for inks, clarifier for liquid systems, etc.
The resulting products produced by both of the foregoing processes have a BET surface area in excess of 100 m.sup.2 /g and a tamped density of approximately 100 g/l. In addition, both of these products have a tendency to gel when in a colloidal dispersion. In order to prevent, to a certain degree, the gelling behavior of a colloidal dispersion of precipitated silica, it has been proposed (Society of Petroleum Engineers Journal, July, 1969, page 42) that precipitated silica be reacted with dimethyldichlorosilane. The precipitated silica produced by this reaction is more expensive because of the additional production step required.
Micro silica is produced as a by-product in the manufacture of ferrosilicon or silicon metals through the use of electric arc furnaces. This by-product material contains high amounts of extremely fine spherical particles of silicon dioxide. The micro silica is captured from the escaping gases of the arc furnaces by means of electrostatic precipitators The collected material generally contains more than 75 percent silicon dioxide. Other constituents are carbon, sulfur, and the oxides of iron, aluminum, calcium, magnesium, sodium and potassium. The chemical composition of this by-product varies depending upon the type of metal or alloy being produced For example, the micro silica produced from a furnace being used to manufacture ferrosilicon metal will generally contain more iron and magnesium oxide than from a furnace being used to manufacture silicon metal. Regardless of the type of charge within the furnace, the micro silica produced as a by-product typically has a BET surface area of approximately 20 m.sup.2 /g, a bulk density of approximately 200 g/l and a tamped density of approximately 600 g/l.
It has been found that the addition of micro silica to fresh concrete affects the pore size distribution and the hydration products formed during hydration, and thereby increases the strength and durability of the resulting concrete product. The properties of micro silica and its use as an additive to cement, concrete and mortar have been thoroughly discussed in a technical paper entitled "Silica Fume" by V. M. Malhotra and G. G. Garette in the May, 1982 issue of Concrete Construction, pages 443 to 446. The technical information contained in the foregoing paper is incorporated by reference in the description of the present invention.
Since micro silica consists of very fine vitreous particles, most of which are smaller than one micron, it has been noted by the aforementioned authors that the use of this product presents handling problems and may pose a health hazard. These problems can be overcome by transporting and using micro silica in the form of an aqueous dispersion consisting, in general, of approximately 50% by weight of water, and the balance being micro silica. A problem, however, arises with the use of such aqueous dispersions of micro silica due to the fact that such dispersions have an affinity to become thixotropic.
The affinity to thixotropy may be desirable in certain applications. For example, this property is desirable and necessary in order to produce the coating of fumed silica or precipitated silica which is placed on cardboard to reduce warpage and to maintain the non-skid properties of same. In the concrete industry, however, the affinity of such dispersions to become thixotropic is undesirable and there has been considerable research with respect to techniques for delaying the thixotropic reaction which occurs in such dispersions.